A temperature compensated crystal oscillator (TCXO) is used for various electronic equipments, such as a communication equipment or an information technology equipment.
FIG. 1 is a cross-sectional view of a conventional temperature compensated crystal oscillator. The conventional temperature compensated crystal oscillator includes a quartz resonator 20 and an integrated circuit (IC) 30, which are arranged in the interior of a housing 1. The quartz resonator 20 has, as illustrated in FIG. 1, a piece of quartz (quartz chip) 2 and a pair of excitation electrodes 2A and 2B connected to the quartz chip 2.
The housing 1 is made of a ceramic and has a lid part 1A. The quartz resonator 20 and the IC 30 are situated inside the housing 1 and the housing 1 is sealed with a desiccated nitrogen gas filled inside the housing 1. The temperature compensated crystal oscillator is mounted to a printed circuit board 5 by attaching the bottom of the housing 1 to the printed circuit board 5.
The quartz chip 2 of the quartz resonator 20 is connected to an inner wall 1B of the housing 1 so that the quartz chip 2 is located at a center of the interior space of the housing 1. For example, the quartz chip 2 is formed by AT cutting to have a thickness with which a desired resonant frequency is obtained. The excitation electrodes 2A and 2B are formed on opposite surfaces of the quartz chip 2, respectively. Thin-film electrodes made of gold (Au) are used as the excitation electrodes 2A and 2B.
The IC 30 is arranged in the bottom part of the interior space of the housing 1. A temperature sensor 4 is attached to the IC 30 to detect a temperature of the IC 30. The temperature sensor 4 is a temperature-detecting device of which resistance value varies in response to a temperature of the IC 30. For example, a nichrome wire can be used as the temperature sensor 4.
FIG. 2 is a circuit diagram of the conventional temperature compensated crystal oscillator illustrated in FIG. 1. The IC 30 includes a variable capacitor 31, an inverter 32, an output buffer circuit 33, a compensation circuit 34, and a memory 35.
The variable capacitor 31 and the inverter 32 are connected to the excitation electrodes 2A and 2B of the quartz resonator 20 so as to form a loop form oscillator circuit containing the quartz resonator 20.
The output buffer circuit 33 changes an oscillation signal acquired by the oscillation circuit into a clock signal, and outputs the clock signal to an external part. Although the output buffer circuit 33 includes a plurality of inverters in many cases in practice, only one inverter is illustrated in FIG. 2 for the sake of simplification of explanation.
The variable capacitor 31 is a variable capacitance element of which capacitance is variable. The variable capacitor 31 is inserted into the oscillator circuit in series in order to make the electrostatic capacitance value of the loop-form oscillator circuit variable. The variable capacitor 31 is formed using a variable-capacitance capacitor such as a varicap diode, and the electrostatic capacitance thereof is variable in response to a voltage applied by a compensation circuit 34.
The memory 35, which is incorporated in the IC 30, stores data representing a reverse characteristic of a frequency temperature characteristic of the quartz resonator 20. The data is used for converting an electric current value representing a temperature signal into an electric current value supplied from the compensation circuit 34 to the variable capacitor 31. The compensation circuit 34 applies a voltage to the variable capacitor 31 in response to the temperature signal (electric current value) representing a temperature detected by the temperature sensor 4 by referring to the data stored in the memory 35. The compensation circuit 34 has a circuit structure such as, for example, disclosed in Japanese Laid-Open Patent Application 2008-300978 (particularly, FIG. 3).
According to the above-mentioned structure, when the temperature detected by the temperature sensor 4 changes, the electrostatic capacity of the variable capacitor 31 is adjusted, and, thereby, the oscillation frequency is controlled to be constant against the temperature change.
The clock signal output from the output buffer circuit 33 of the temperature compensated crystal oscillator is used by a central processing unit (CPU) or a communication part contained in electronic equipments.
Additionally, Japanese Utility-Model Registration No. 2503834 discloses a related art.
With miniaturization of electronic equipment, high-density mounting is achieved especially in electronic equipment using a temperature compensated crystal oscillator. Particularly, high-density mounting is performed in portable equipment such as a cellular phone.
In order to realize such a high-density mounting, design flexibility in the interior of electronic equipment is restrained greatly. There is a case where a temperature compensated crystal oscillator must be mounted near a heat-generating part such as a transmitting unit in a cellular phone.
In the case of the transmitting unit of a cellular phone, a temperature may sharply increase when performing communication. A large part of heat generated in the transmitting unit is transferred to a temperature compensated crystal oscillator through a board on which the transmitting unit is mounted. If a temperature sensor is located near the board as is in the above-mentioned conventional structure, a temperature of the temperature sensor rises faster than a quartz resonator. Thus, a difference is generated between the temperature detected by the temperature sensor and the temperature of the quartz resonator. This results in inaccurate temperature compensation, and a frequency output from the temperature compensated crystal oscillator is changed undesirably. In order to eliminate such a problem, it is considered to attach the temperature sensor directly to the quartz resonator.
However, if the above-mentioned sharp temperature rise occurs, the quartz resonator is heated rapidly. Thus, it may be difficult to follow the temperature change even if the temperature change in the quartz resonator is detected by the temperature sensor attached to the quartz resonator.